The present invention relates to tubular ceramic articles comprised of silicon and silicon carbide, and to processes and apparatus for the manfacture of such articles.
Silicon carbide, a crystalline compound of silicon and carbon, has long been known for its hardness, strength, and excellent resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good heat transfer properties and exhibits high strength and excellent creep resistance at elevated temperatures. These desirable properties may be attributed to a strong covalent chemical bonding, which also is the cause of an undesirable property of silicon carbide, that of being difficult to work or fabricate the material into useful shapes. For example, because silicon carbide dissociates at high temperatures, rather than melting, it is not feasible to fabricate articles by melt processes, and because silicon carbide has a very slow diffusion rate, fabrication by plastic deformation processes is not viable.
It has been proposed to produce shaped silicon carbide articles by forming bodies of silicon carbide particles and either bonding or sintering the particles at high temperatures to form a consolidated body. If the particulate silicon carbide starting material is fine enough, and suitable sintering aids are added, the fine, particulate material will exhibit sufficient self-diffusion at high temperatures that the particulate material will sinter and form into a substantially dense single phase material. Sintering processes, in general, require fine powder starting materials and pressureless sintering processes, in particular, require an even finer starting material. Because of the needed fineness and high purity of the starting materials, articles formed by sintering processes are relatively expensive.
Coarser and less pure silicon carbide powders are known to bond together at high temperatures. However, the resultant products have considerable porosity and for that reason are usually not as strong, or as corrosion resistant, as more fully densified materials. The properties of such materials may be substantially improved by infiltrating the pores of such materials with silicon, in either vapor or liquid form, to produce a two phase, silicon-silicon carbide product. Although such processes utilize relatively inexpensive coarse powders as starting materials, they require two high temperature furnacings, one to form the silicon carbide to silicon carbide bond and a second, separate furnacing, to infiltrate the formed body with silicon.
Mixtures of coarser and less pure silicon carbide powders with particulate carbon or with a carbon source material may be preformed and subsequently impregnated with silicon at high temperature to form "reaction bonded" or "reaction sintered" silicon carbide products. The carbon component may be in the form of particulate graphite or amorphous carbon, or may be in the form of a carbon source material, for example a carbonizable organic material, such as, pitch, resin or similar materials, which will decompose during furnacing to yield carbon. The infiltrating silicon reacts with the carbon in the preformed body to form additional silicon carbide which bonds with the original silicon carbide particles to produce a dense silicon carbide article. Typically reaction bonded silicon carbide materials are characterized by almost zero porosity and the presence of a second phase, or residual, of silicon, usually greater than about 8% by volume.
In typical siliciding or typical reaction bonding processes, the particulate silicon carbide and carbon starting material is initially preformed or preshaped into an article, commonly referred to as a "green body", which is subsequently fired. The particulate silicon carbide and carbon starting mixture is commonly blended with a binder to aid in shaping. If the binder is dry, or relatively dry, the powder may be compacted to the desired shaped green body using a press or isopress. If the binder is liquid, or semi-liquid, and is used in sufficient quantity, the mixture may suitably be slip cast, extruded or injection molded to form a shaped green body.
High temperature heat exchange components desirably have relatively thin walls to facilitate high rates of heat transfer. There have been previous attempts to fabricate tubular articles of silicon carbide by various methods, however, none have proven commercially successful. For example, U.S. Pat. No. 801,296 discloses a method of producing a hollow silicon carbide tube by siliciding a solid carbon rod to form an outer layer of silicon carbide and subsequently burning out the carbon interior leaving the outer layer of silicon carbide. U.S. Pat. No. 1,266,478 describes a typical method of preforming a tubular body of silicon carbide and carbon and siliciding to obtain a tubular silicon carbide article. U.S. Pat. No. 1,756,457 teaches the reaction of silicon dioxide and carbon in preformed columns to produce a silicon carbide tube. U.S. Pat. No. 3,495,939 teaches making tubular silicon carbide by preforming a tube of particulate silicon carbide and carbon, positioning the tube vertically in a furnace and siliciding with the bottom of the tube in contact with liquid silicon. U.S. Pat. No. 3,882,210 teaches siliciding a preformed tube of alpha silicon carbide and graphite to produce a tube of silicon carbide. U.S. Pat. No. 4,265,843 describes the manufacture of silicon carbide in tubular form by initially heating at low temperature a rotating preformed carbon tube in the presence of silicon to impregnate the tube and subsequently heating at a higher temperature to react the silicon and carbon to form a tube of silicon carbide.
It will be appreciated that the fabrication of long, (e.g., four to eight foot), large diameter, (e.g., four to eight inch OD), thin-walled, (e.g., 1/8 to 1/4 inch), tubes presents a difficult problem. The tubular green bodies that are required to be initially formed by the prior art processes are inherently structurally weak and easily deformed or broken unless handled with utmost care. In subsequent processing steps, the tubular green bodies must be carefully dried, and/or baked, and positioned in a furnace for siliciding. The fragility of the preformed bodies and the required multiple handling entailing high labor imput have been major factors in preventing the use of tubular silicon carbide in many applications purely on the basis of cost.
The term "reaction sinter" as used herein means consolidation by chemical reaction and includes the reaction of silicon with carbon either alone or in mixture with silicon carbide.
The term "carbon" as used herein means carbon or a carbon source material that produces carbon upon heating that will react with the infiltrating silicon to form additional silicon carbide, in situ.
The term "tubular" as used herein means that the article has the form of a tube, that is, it is fistulous. Although the present invention will hereinafter be described in terms of tubes having generally round cross-sections, it will be understood that the invention is not so limited and that tubes having eliptical, square or multi-sided cross-sections, or having an external surface of one cross-sectional type and an internal surface of another, may as easily be produced. It will also be understood that the present invention also contemplates tubular articles that have internal separations, or septums, providing multiple passageways within the tube.